drm/ttm/tests: Add tests for ttm_pool

Add KUnit tests that exercise page allocation using page pools
and freeing pages, either by returning them to the pool or
freeing them. Add a basic test for ttm_pool cleanup. Introduce
helpers to create a dummy ttm_buffer_object.

Signed-off-by: Karolina Stolarek <karolina.stolarek@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/427ea373357d0b6cb376c9d7ebc33c930bf1d28a.1691487006.git.karolina.stolarek@intel.com


Signed-off-by: Christian König <christian.koenig@amd.com>